He led the way to his study, which since the destruction of the observatory had been converted into a temporary laboratory.
Ten minutes later, Professor Wentworth had his re-developing bath ready in a porcelain basin and had plunged some of the negatives into it.
“This process is what photographers call intensification,” he explained. “It consists chemically in the oxidation of a part of the silver of which the image is composed. I have here in solution uranium nitrate, plus potassium ferricyanide acidified with acetic acid. The latter salt, in the presence of the acid, is an oxidizing agent, and, when applied to the image, produces silver oxide, which with the excess of acetic acid forms silver acetate.”
“Which is all so much Greek to me!” said Carter.
“At the same time, the ferricyanide is reduced to ferrocyanide,” the professor went on, with a smile at Joan, “whereupon insoluble red uranium ferrocyanide is produced, and, while some of the silver, in being oxidized by this process, is rendered soluble and removed from the negative into the solution, it is replaced by the highly non-actinic and insoluble uranium compound.”
The process was one quite familiar to photographers experienced in astronomical work, he explained. In fifteen minutes they should know what results they were getting.
But when fifteen minutes passed and the negatives were still as black as ever, Jim’s hope waned.
Not so Professor Wentworth’s, however.
“There is a definite but slow reaction taking place,” he said after a careful examination. “Either the over-exposure is even greater than I had suspected, or the actinic rays from your interesting subjects have formed a stubborn chemical union with the silver of the image. In the latter event, which is the theory I am going to work on, we must speed up the reaction and tear some of that excess silver off, if we’re ever to see what is underneath.”